An injector of this type is already known from German Patent application DE 199 40 055 C1.
As is generally known fuel injection valves equipped with piezoelectric multilayer actors can switch more quickly compared to conventional, electromagnetically activated fuel injection valves. However the design of an injector equipped with a piezoelectric actuator must take account of the fact that heat is lost through losses within the body of the actor and this must be removed so that the actor does not overheat. This heating up can damage or destroy the actor through thermal expansion of the actor body. On the other hand it is possible through the additional internal heating up of the actor body for the Curie temperature to be exceeded when the internal combustion engine is working at a high temperature level, since the direct injection into the combustion chamber subjects the injector to high ambient temperatures right from the start.
Impermissibly approaching or even exceeding the Curie temperature must be prevented in any event since otherwise there is the danger of the piezoceramic depolarizing and the actor losing lift.
Although the present invention is applicable to any injector with piezo actor and intermediate space (from actor to injector housing) it becomes particularly significant in relation to applications in which, for operation of high-pressure injection valves for direct fuel injection with a piezoelectric multilayer actor (PMA) as drive element—both for diesel and for gasoline engines—the aim is for multiple injection to optimize the combustion process. With the diesel engine pilot injection achieves a conditioning of the mixture so that after the main injection there is an even combustion process. With the gasoline engine on the other hand a leaner mixture will be more safely ignited by an explicit secondary injection enriching the mixture in the area of the spark plug.
Further development is generally moving in the direction of continuous injection rate forming, to further improve consumption and exhaust gas values and to reduce the noise generated. Concepts with up to five injections per combustion process have already been discussed. Accordingly the actor is to be activated with an ever higher frequency, whereby however, in the piezoceramic of the actor, as previously described, even more waste heat will then be produced. This waste heat can currently not be very well removed since the piezoceramic is typically surrounded by air so that heat can basically only be effectively removed directly or indirectly via the ends of the actor body.
A dosing valve with piezo actor is known from DE 199 40 055 C 1 cited at the start, in which the actor space (intermediate space), of a hydraulic chamber and an equalization space are hydraulically linked and filled without bubbles with a pressurized hydraulic fluid, in order to form a dynamic (i.e. with the given injection times in the milliseconds range) rigid support for the piezo actor and a hydraulic length equalization element for longer lasting processes. In this connection actors of the “closed” type, in which the actor is encapsulated by a metal bellows positioned in the space, as well as actors of the “open” type are discussed. The patent application mentions that with a more expensive “open” special version compared to the metal bellows, with an actor incorporated into a tubular spring, by direct contact of the actor with the hydraulic fluid, for example silicon oil, advantageously produces heat dissipation to the environment (not specified in more detail).